Venezuela’s authoritarian leader, Nicolás Maduro, was declared the winner of a high-stakes presidential vote by the country’s election authority early Monday, a result that would extend his party’s rule into a third decade.

The vote was riddled with irregularities, and citizens were angrily protesting the government’s actions at voting centers even as the results were announced.

With 80 percent of voting stations counted, the election authority claimed that Mr. Maduro had received 51.2 percent of the vote, while his closest competitor, Edmundo González, had received 44.2 percent.

The result is very likely to be disputed by the country’s opposition, which for the first time in years believed it had a shot at ousting Mr. Maduro at the ballot box. Frustration over the outcome could plunge the oil-rich, crisis-laden nation into a period of deep uncertainty, with concern that street demonstrations could follow.

In the past, security forces aligned with Mr. Maduro have crushed protests with violence. The election body that declared the victory is led by a longtime ally of the president.

During Mr. Maduro’s party’s 25 years in power, it has presided over an economic contraction unlike any seen outside war and has become the source of one of the largest migrant crises in the world.